Table of ContentsIntroductionOverviewnanoIR Platform nanoIR Setup Features of nanoIR PlatformMeasurements of Drop-Cast P3HT and PCBMdoped P3HT films Drop-Casted P3HT Film PCBM-doped P3HT blendConclusionAbout Anasys Instruments
Organic photovoltaic (PV) materials are used in harnessing solar power as an alternative energy source. Polymer blends of poly(3-hexylthiophene), P3HT, and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) are a popular donor-acceptor (DA) bulk heterojunction (BHJ) that are widely used for such applications. AFM and TEM have been used to characterize the structure of the PV films at high-spatial resolution; but chemical information is very difficult to obtain at the nanoscale.
In this application note, topographical features have been correlated to local chemical spectroscopy on P3HT and PCBM- doped P3HT films using the innovative nanoIR™ technology. A set of high spatially (~ 100 nm) resolved chemical analysis of photovoltaic materials, namely, P3HT (poly(3- hexylthiophene)) and PCBM ((6,6)-phenyl-C61-butyric acid methyl ester) are performed.
Infrared (IR) spectroscopy is commonly used for analytical measurement in industrial and academic R&D laboratories. The spatial resolution breakthrough is obtained by an innovative technique that uses a nanoscale probe from an atomic force microscope (AFM) that acts as the IR absorbance detector. The nature of the IR absorbance detection results in measurements of nanoscale mechanical properties simultaneously with nanoscale morphology, along with chemical composition. The nanoIR also has integrated nanoscale thermal property mapping resulting in a multifunctional tool that provides nanoscale structure, chemical, mechanical and thermal properties. Dr. Alexandre Dazzi from the Laboratoire de Chimie Physique, CLIO, Universite Paris-Sud, Orsay, France, pioneered a patented technology that combines AFM and IR Spectroscopy (AFM-IR).
Figure 1. The nanoIR Platform
Figure 2. Close up view of the prism and AFM measurement head
The nanoIR system uses a pulsed, adjustable IR source to excite molecular vibrations in a sample that has been mounted on an IR transparent (ZnSe) prism. The system’s IR source is developed using a proprietary technology which is continuously adjustable from 1200 to 3600 cm-1 covering a broad range of the mid-IR spectrum. The absorption of radiation by the sample results in heating and rapid thermal expansion that causes resonant oscillations of the cantilever. The induced oscillations result in a characteristic ringdown as shown in Figure 3.
Figure 3. Schematic showing the technique behind the nanoIR
It is possible for nanoIR users to rapidly survey regions of a sample through AFM imaging and then obtain high resolution chemical spectra at selected regions on the sample. As shown in Figure 4, polymer spectra obtained from the nanoIR system have demonstrated good correlation with bulk Fourier transform infrared (FT-IR) spectra.
Figure 4. A comparison of the spectrum generated by the nanoIR (red) and conventional FT-IR (blue) of a polystyrene sample.
Features of nanoIR Platform
The features of the nanoIR platform are listed below:
- The nanoIR system provides high-resolution infrared spectra, and data on the mechanical properties of the sample. This is accomplished, as mentioned above, by monitoring the frequency of the fundamental or higher resonant modes of the cantilever.
- The contact resonant frequency of the cantilever is directly related to the stiffness of the sample and can be used to map the modulus of the sample qualitatively.
- The nanoIR platform can also perform nanoscale thermal analysis utilizing novel AFM cantilevers that integrate a resistive heating element at the cantilever end
- Using these cantilevers along with the system allows the local measurement of the transition temperature of materials at one point or an array of points across a sample.
- This allows detection or mapping of the amorphous/crystalline content, stress, extent of cure, or other material characteristics which can be characterized by the transition temperature of the material.
Measurements of Drop-Cast P3HT and PCBMdoped P3HT films
The nanoIR technique is perfect for measurement of polymeric samples in which there are local material variations. According to the sampling technique, the material has to be deposited as a thin film on a ZnSe prism. Hence the materials are subjected to dropcasting from solution directly onto the prism.
Drop-Casted P3HT Film
It is important to note that not all surface features on the same AFM image share identical infrared absorption characteristics. An area of interest is shown in Figure 5 where tiny protrusions of the order of few microns are seen.
Figure 5. Point-and-click spectral acquisition over a large area of a thin P3HT film on a ZnSe prism
Normalized point-and-click IR spectral acquisition reveals that only some points have slightly broadened absorption features such as a long absorption tail (Spectrum 10) and the shoulder near 1500 cm- 1 is less defined. At the other points, it is found that spectra yielded are similar to the bulk P3HT material. At points related to spectrum 12 to 14, it is seen that the broadened absorption appears to be away from the height feature. To improve the spectral analysis, the region near the spectrum 12 to 14 was scanned again with a higher spatial resolution and the related image is shown in Figure 6(top) and the spectral array acquisition obtained afterward is shown in Figure 6 (below).
Figure 6. A spectral array acquisition showing the AFM image (top) and the corresponding spectra (bottom) near spots 12-14 in Fig 5; spacing between each marker is ~ 100 nm
The spectra are observed approximately 100 nm apart and spectral variations are seen within the same length scale (from second to third and from fifth to sixth spectra). As the shoulder around 1500 cm-1 vanishes and then again appears at the arrows, the signal near 1380 cm-1 appears to widen. By using the nanoIR™, these IR spectral changes can be seen at an amazingly high spatial resolution.
PCBM-doped P3HT blend
In this example, a surface defect is observed in Figure 7, which shows an AFM image of a heat treated P3HTPCBM sample. The localized IR spectra specific to surface features are shown directly below the image.
Figure 7. An AFM image and the spectra of a heat-treated PCBM-doped P3HT sample
When the nanoIR spectra is compared with the nanoIR spectra for the pure components, local alterations are identified. The methylene bending modes at 1444 cm-1 and 1432 cm-1 corresponds to the P3HT and PCBM, respectively. The 1444 cm-1 band also has a contribution from an overlapping ring semicircle stretching mode. The corresponding spectrum for the yellow hash mark has both components. At the outer ring or the red mark or spectrum 1, the peak at 1732 cm-1 (PCBM) is tiny and the component at 1444 cm-1 (P3HT) is dominant. At both green and purple hash marks (spectra 3 and 4), the band near 1432 cm-1 is primarily contributed by PCBM. Finally, the sharpness of the band at 1432 cm-1 and a stronger 1732 cm-1 signal suggest the lobe at the center is mostly PCBM.
The stiffness of the surface defect in relation to P3HTPCBM blend can be imaged using the nanoIR™. When exposed to a continuously-pulsing IR laser radiation at 1450 cm-1 the contact frequency of the cantilever is traced constantly as the AFM tip moves across the sample. Here the bulk material (yellow/orange) appears stiffer than most of the interior areas of the defect (green).
Figure 8. Contact frequency image of a chemical defect mapped over the corresponding height image; the range of the frequency is approximately 30 kHz (color bar: orange . stiffer; deep brown - softer)
The data obtained from the analysis show the capability of the nanoIR™ to analyze a set of photovoltaic materials with high spatial resolution (~100 nm). The topological features can be linked to their corresponding chemical infrared signatures. 100 nm spatial resolution can be easily achieved in applications where domain boundaries are not known. Local phase separation of materials are found by comparing local nanoIR pectra at the defect sites with the bulk spectra of the pure components. In addition, the relative contact frequencies surrounding the defect are mapped simultaneously with the corresponding topography.
About Anasys Instruments
Anasys Instruments Corporation is the pioneer in the field of sub-100nm thermal property information. The Company's technology and products are being used to address metrology and analysis challenges in the polymers, pharmaceuticals, data-storage, and advanced-materials markets. In 2007, Anasys was named as winner of two prestigious industry awards, the R&D 100 Award and the inaugural MICRO/NANO 25 Award, both of which recognise Anasys as leaders in innovative technology.
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